safety. maybe you need to get down in a hurry. so you pull a cord and the hole opens up, some of the hot air escapes, and you can make a rapid descent without having to rely on the air to cool down first. otherwise its there to make controlled descents. read more
Best Answer: For the same reason why warm air currents rise, causing 'Thermals' (decrease in air density causing lift), upon which gliders,hang gliders and birds can soar upwards. The mechanics of the hot air balloon is, in effect, produced by burning propane and trapping the resultant thermal in the balloon which soars upwards controlled by the amount of propane being burned. read more